Images.gmk defines the following logic for the db component:

ifndef OPENJDK
  DB_ZIP_DIR := $(wildcard $(JDK_TOPDIR)/src/closed/db)

  $(IMAGES_OUTPUTDIR)/_unzip/%.unzipped: $(DB_ZIP_DIR)/%
        $(ECHO) Unzipping $(patsubst $(SRC_ROOT)/%,%,$<)
        $(RM) -r $(JDK_IMAGE_DIR)/db
        $(MKDIR) -p $(JDK_IMAGE_DIR)/db
        cd $(JDK_IMAGE_DIR)/db && $(UNZIP) -q -o $< -x */index.html */KEYS */test/* *javadoc/* */docs/* */demo/* 2> /dev/null
        cd $(JDK_IMAGE_DIR)/db && $(MV) db-derby-*-bin/* . && $(RM) -r db-derby-*-bin
        $(MKDIR) -p $(@D)
        $(TOUCH) $@

  $(JDK_IMAGE_DIR)/db/README-JDK.html: $(DB_ZIP_DIR)/README-JDK.html
        $(ECHO) $(LOG_INFO) Copying '$(patsubst $(OUTPUT_ROOT)/%,%,$@)'
        $(MKDIR) -p $(@D)
        $(CAT) $< | $(SED) "s/XXXX/$(shell cat $(DB_ZIP_DIR)/COPYRIGHTYEAR)/" > $@

  $(JDK_IMAGE_DIR)/db/3RDPARTY: $(DB_ZIP_DIR)/3RDPARTY
        $(ECHO) $(LOG_INFO) Copying '$(patsubst $(OUTPUT_ROOT)/%,%,$@)'
        $(MKDIR) -p $(@D)
        $(CAT) $< | $(SED) "s/XXXX/$(shell cat $(DB_ZIP_DIR)/COPYRIGHTYEAR)/" > $@

  JDK_DB_TARGETS := $(patsubst $(DB_ZIP_DIR)/%, $(IMAGES_OUTPUTDIR)/_unzip/%.unzipped, \
      $(wildcard $(DB_ZIP_DIR)/*.zip)) \
      $(JDK_IMAGE_DIR)/db/README-JDK.html $(JDK_IMAGE_DIR)/db/3RDPARTY

endif


Note that the first target will delete the $(JDK_IMAGE_DIR)/db directory, while the following targets will try to copy files into it. There are no dependencies listed between these tasks and so they can run concurrently and hence conflict. This has been observed in JPRT builds.